1. Field of the Invention
The present invention relates to a direct spread type, spread spectrum signal receiving apparatus, and more particularly, to a spread spectrum signal receiving apparatus which prevents a phase error between a PN (pseudonoise) code on the transmitter side and a PN on the receiver side when a phase locked loop is used to acquire and maintain synchronism.
2. Description of the Prior Art
As a type of radio communication system, the spread spectrum communication system is widely known. In this spread spectrum system, the transmitter side modulates the carrier by an information signal, such as an audio signal or data, and the modulated carrier by the information signal is then multiplied by a spreading code such as an M-sequence or the like to produce a spread spectrum signal. The spread spectrum signal is transmitted from an antenna. The receiver side despreads the received spread spectrum signal by multiplying it by the same spreading code which was used on the transmitter side. The despread signal is then demodulated to obtain the original information signal.
In spread spectrum communication systems, as described above, when despreading the received spread spectrum signal, it is necessary to synchronize the spreading code generated on the receiver side and the spreading code contained in the received signal. Accordingly, a spread spectrum signal receiving apparatus designed to maintain synchronization between the spreading code generated on the receiver side and the spreading code in the received signal has been proposed.
In FIG. 1, the spread spectrum signal is changed by a frequency converter 1 to a low frequency bandwidth for ease of processing in the subsequent circuits, then the spread spectrum signal is multiplied by a spreading code, generated by a spreading code generator 3, in a multiplier 2. The phase of the output signal of the multiplier 2 is compared, by a phase comparator 4, with the phase of the output signal of a VCXO (voltage-controlled crystal oscillator) 5. The output signal of the phase comparator 4 is smoothed by an LPF 6, then applied as a control signal to the VCXO 5, so that the oscillation frequency of the VCXO 5 is varied according to the control signal. While supplying its output signal to the phase comparator 4, the VCXO 5 also supplies its output signal, through a frequency divider 7, to the spreading code generator 3. Those circuits, that is to say, the phase comparator 4, the LPF 6, the VCXO 5, constitute a so-called PLL (phase-locked loop), and the PLL operates such that the phase difference of the two input signals to the phase comparator is held at zero. According to changes in the oscillation frequency of the VCXO 5, the PLL operates to synchronize the phases of the two input signals to the phase comparator 4, and as a result the phase of the output signal of the multiplier 2 and the phase of the output signal of the VCXO 5 are synchronized.
After the phase has been locked by the PLL, a spreading code which is in phase with the spread spectrum signal is generated, and the spread spectrum signal is despread as the spread spectrum signal is multiplied by the spreading code in the multiplier 2. The output signal of the multiplier 2, derived from despreading, is applied through a BPF 8 to a demodulator 9, which demodulates to obtain the information signal.
In FIG. 1, the PLL operates to synchronize the two input signals to the phase comparator 4, so that the phase difference between the output signal of the multiplier 2 and the output signal of the VCXO 5 becomes zero. However, in actuality, due to delays in the circuit devices in FIG. 1 or otherwise, the phase of the spreading code generated on the receiver side and the phase of the spreading code in the spread spectrum signal do not precisely coincide with each other, and therefore despreading is not always accurate. This problem is referred to as step-out.